How to MegaSquirt your Subaru RX Turbo

Building a Harness Adaptor to use the stock wiring harness for full control
of the stock fuel and ignition systems with the MegaSquirt-II

Updated 3/1/2006

Photo by John Swain

Application: 1987 Subaru RX Turbo -- Other
surrounding year models are very likely the same, check ECU pinout and
distributor part# to compare.

Level of control: Full Standalone EMS-- Fuel and Ignition
Control

So you've opened up the latest edition of SubieSport Magazine, thumbed
through it and read up about this MegaSquirt thing and our little adventures of
installing and dialing in a Megasquirt-II on Will Maham's Scooby RX Turbo.
The wiring portion of this car is really pretty straightforward, we had to play
around with the ignition settings within the software a bit to get it all right
and then spent a bit of time on the dyno but our results ended up looking pretty
good I think considering we made no other changes and still made 10hp and 8lbs
of torque at the wheels peak, and a healthy increase across the band-- before
turning up the boost... see below for full dyno results.

So without further ado here's the wiring diagram for doing the exact same
thing to your own Subaru RX Turbo--

This guide will allow you to build your own 'harness adaptor' to do a
Plug-n-play style installation of the MegaSquirt-II ECU into your Subaru RX
Turbo that will look something like this (before you wrap it up and make it
pretty):

You can see the gray wire looped around on the larger connector from one pin
to another, if you look up at the schematic above this is the wire labeled '12v
power source to Optical CAS in Dizzy'. You can also see that we used this
12v source to connect one end of a 1k 1/4watt resistor to, and connected the
other end to a bit of wire running to pin 17 of that same large connector, also
shown on the schematic above. This is the 'TACH Pullup' which 'pulls' the
Optical CAS sensors output up to 12v so that the MegaSquirt can read it
properly. It's looking for a 12v square wave on the ignition input (TACH
input--pin24 to the MS-II).

You can source your own wire to build this out if you'd like, or you can use
one of my
universal wire harnesses as a basis to provide the color coded (hi-temp
automotive grade) wiring already attached to the MegaSquirt ECU connector, then
you just need to dig up the Subaru ECU connector from a junkyard ECU and solder
up the one side. For Will's car I used some spare wire I had laying around
the shop instead of one of my harnesses as I don't mind soldering all the wires
to the connector and I had to use that wire for something....

And the only bit of underhood wiring is: You need to mount an IAT
sensor in your intake tube just before the throttle body and run the two wires
into the stock MAF connector where you've unplugged the MAF. Here's a
picture of how to wire it into the MAF connector, the sensor isn't polarized so
which wire goes into which MAF connector hole isn't important, just that you
hook them into the connector as seen above. This will route the IAT sensor
signal through your stock wiring harness, back to the 'harness adaptor' you
built, and to the MegaSquirt-II.

Here is a link to my Wiring &
Sensors catalog where you can find the IAT sensor and a weld-on bung you can
use to fit it to your intake tube.

MegaTune Configuration

Note that your mileage may vary with this configuration info-- the core of it
should be pretty accurate for a similar vehicle, basically everything except the
VE table and the Ignition Table. Those tables you'll want to go over and
dial in for your specific ride, these will likely be pretty close if you car's
mods are similar to Will's. Pretty much stock motor with stock injectors,
stock turbo, aftermarket downpipe and exhaust, A2W Intercooler, etc. Even
if your car is an exact duplicate... take the time to make sure the map is
right.... it's your responsibility to tune it in-- i'm trying to take care of
some of the guesswork for you though ;). This is what worked on Will's
car:

One more note-- we never went over 10-11psi even though these tables are
setup for much more, keep that in mind, we've tuned nothing above 10-11 psi
(about 170 kpa) so that info is a total guesstimate up above there...

Dyno Results

Click the dyno graphs for larger images

This first map compares the stock to stock, only change is the ECU and tuning
of the Megasquirt-II EMS. The peak numbers tell a bit of the story, with a
little over 10hp and 8lbs of torque at the wheels, but the real benefit is in
the broad gains all across the powerband where's there's a 5 to as much as a
17hp gain around 5200rpm. And similarly a 5-15 ft-lb torque bump through
much of the range... this made all the difference in the world on the autocross
course, where peak peak power means very little (still nice to have gained there
though), and broad torque means everything.

This second map also has the stock to stock comparison, but adds a 10-11psi
run after we installed a MBC and turned it up a bit. Here we made an
additional 13.6whp and 16.5 ft-lb of torque on top of what we made at 7psi, and
you can see we worked on our tuning a bit more and got that nasty dip at 2500
tuned out as well. And you want to talk about torque increase across the
band? Compared to stock we're now looking at a 24whp and 15 ft-lb increase
starting at 2900rpms when the turbo has finished spooling up, and staying pretty
solid increasing to more than a 40 whp and 40 ft-lb increase just above 5000rpm
and carrying most of that all the way to redline... Too bad Will couldn't
take it too the track this way-- he had to drop it back to 7psi for his class,
but it was fun on the street for a bit...